Cathode ray tube circuit



Nov. 7, 1939. A. J. YOUNG GATHODE RAY 'fuBE CIRCUIT Fild Feb. 23, 1935 2 Sheets-Sheet 1 INVENTOR ARTHUR J. YOUNG .zg wm/ ATTORNEY Nov. 7, 1939. A. J. YOUNG 2.179.111

CATHODE Rm TUBE CIRCUIT Filed Feb. 23, 1935 2 Sheets-Sheet 2 I Y 1k ,2

lllllAll INVENTOR ARTHU J OUNG B //Lov-w I ATTORNEY Patented Nov. 7, 1939 E3 gTATES CATHQDE RAY TUBE CIRCUIT Arthur James Young, London, England, assignor to Radio Corporation of America, a corporation of Delaware Application February 23, 1935, Serial No. 7,736 in Great Britain February 23, 1934 8 Claims.

This invention relates to cathode ray tube circuitarrangements and more specifically to circuit arrangements embodying cathode ray tubes and wherein it is required that the cathode ray be modulated in intensity and the modulated ray caused to scan a predetermined area. For example, in cathode ray tube television receivers wherein the received picture is built up upon a fluorescent screen at the end of the tube, it is required that the spot of light caused by the incidence of the ray upon the screen be modulated in intensity in accordance with received signals and at the same time scan the said screen so as to build up a received picture in the Well known way.

It is quite common in circuit arrangements of the kind in question to modulate the cathode ray by application of modulation potentials to a grid electrode forming part of the electron gun of the tube. There are certain practical objections to the so-called grid method of modulation. Among the objections to this method of control are (1) that the size of the scanning spot depends'upo-n the cathoderay current and (2) that the scanning spot tends to shift its position on the screen by reason only of modulation grid voitage, that is to say, when the grid potential is varied for modulation purposes the efiect is not only to vary the intensity of the ray, but also to vary the position of the light spot. As will readily be appreciated by those skilled in the art, both these matters are somewhat serious in connection with television reception and tend to prevent the obtaining of satisfactory pictures.

Another known method of modulating a cathode ray in a cathode ray tube is by varying the anode potential of the tube, as for example shown in U. S. Patent No. 1,793,496, issued to Kell on February 16, 1931, and entitled Transmission of pictures or views, but this method as practiced heretofore also has a serious disadvantage, namely that since the sensitivity of the customary deflecting means provided in the tube depends upon the anode voltage, i. e. the higher the anode voltage the less'the deflection caused by a given potential applied to the deflecting means, the result of modulating the cathode ray beam by varying the anode voltage only, is to produce an additional undesired modulation of the speed of movement of the scanning spot. This effect is undesirable because the mean value of the modulating input may not be zero, whence it follows that the length of the scanning lines may not be constant.

An important object of the present invention, therefore, is to meet the above mentioned diificulties and to provide an anode modulation system in which undesired modulati'onof scanning speed shall be substantially reduced or eliminated.

According to this invention a cathode ray tube circuit arrangement comprises meansfor modulating anode voltage in the tube in dependence upon modulating potentials and means for simultaneously varying,also in dependence upon modulating potentials, the E. M. F.s applied to the deflector means provided for scanning purposes, the

arrangement being. such that variations in the scanning speed, as a result of anode modulation, are substantially off-set by. the compensating variation in scanning speed due to modulation of deflecting potentials.

The invention will be explained further in the description following when read in connection with the drawings accompanying this disclosure. In the drawings:

Fig. 1 shows diagrammatically a circuit arrangement for applying operating potentials to a cathode ray tube (not shown) Figs. 2 and 3 are valve characteristic curves relating to the apparatus of Fig. 1;

Fig. 4 shows. a modification of Fig. 1; and

Fig. 5 shows a still further modification.

.The cathode ray tube (not shown) intended to be employed in co-operation with circuit arrangements in accordance with this invention is assumed to be of the well known type comprising a cathode, a Wehnelt cylinder or like electrode adjacent said cathode, an apertured anode on the sideof the said Wehnelt cylinder remote from said cathode, and two mutually perpendicular pairs of electrostatic deflecting plates arranged in succession along themean' path of the cathode ray; the tube having-a gasfilled envelope with a fluorescent screen at its end as in the usual way. It is assumed that'the tube is to be employed for reconstructing received television pictures.

Referring now to the" drawings and first to Fig. l-thereof picture signals are applied at terminals points i between the control grid and the cathode of a thermionic valve V1, shown as of the pentode type. The valve V1 is provided with an indirectly heated cathode 2, a control grid 3, a screen grid 3, a suppressor grid 5 and an anode t. The anode 6 is connected through a resistance R1 to a point i upon apotentiometer resistance which is connected between the positive and negative terminals of asource of high tension potential. A by-pass condenser 8 is so arranged that one terminal is connected to the point I and the other terminal to earth. A variable tapping point 9 upon the resistance R1 is connected to a resistance R2 across which are applied potentials-from a suitable electrical time base circuit (not shown) which is connected to terminals iii and may be, for example, a periodic generator of the electric gas discharge type. A variable tapping point it upon the resistance R2 is connected to the control grid l2 of a further thermionic valve V2 also of the pentode type. This valve V2, in addition to the control grid 12, is provided with an indirectly heated cathode l3, a screen grid M, a suppressor grid I5, and an anode H3. The valve V2 is preferably of the so-called variable mu type. The anode of the valve V2 is connected through a re-- sistance I"! to the positive terminal -|-l-IT of the source of anode potential, and the bias of the valve V2 is determined by a battery or other source EG2 shunted by the condenser 28 which is inserted in a lead between the cathode E3 of the valve V2 and the anode 6 of the valve V1. The valve V1 receives bias from a suitable source EG1 shunted by the condenser H9.

The circuit adjustments are as follows:-

The valve V1 is biased to cut-ofi so that the anode current of this valve, and hence the voltage drop in resistance R1 will depend upon the signal input at terminals I, the said voltage drop across R1 being fluctuating direct current. The source EG2 is of such value as to bias valve V2 normally to cut-off and this valve V2 is of such nature as to have a mutual conductance characteristic as shown by Fig. 3 of the drawings.

It will be appreciated that the input to the control grid of valve V2 is made up by a fixed direct current bias, a varying direct current bias in correspondence with the signal strength at the terminal points i and a time base signal derived from the apparatus connected at the terminal points it]. The effect of the varying direct current bias is to shift the operating point of the valve V2 from a point given by the line B of Fig. 3 to a point given by the line EVE! of the said Fig. 3. The output from the valve V2 as set up across the resistance H is applied to one pair of electrostatic deflecting plates of the cathode ray tube (not shown), and it is to be understood that signals corresponding to those applied at terminals I and of suitable amplitude are also applied as in the manner usual in anode modulation schemes to the anode of the cathode ray tube, 1. e. between the anode and cathode thereof as shown in the above referred to patent to Kell.

If desired an auxiliary high tension supply may be used in series in conjunction with the anode modulation voltage, being connectedin such manner that the changed sensitvity of the oscillograph due to the anode modulation is substantially compensated. For the arrangement of Fig. 1 this involves that the auxiliary voltage and the modulation voltage should be cumulative in effect but for the arrangement of the accompanying Fig. 4 (to be described later and wherein there is concurrent increase of modulating voltage and decrease of correcting voltage) the anode and auxiliary voltages should be applied in opposi- I tion. As will be better appreciated after the description relating to the said accompanying Fig. 4 has been read, if the grid and cathode connections, of the valve V2 of the said figure were reversed the auxiliary voltage would be applied in the same sense as for Fig. 1. With the arrangement shown in the accompanying Fig. 5 (to be described later) the auxiliary voltage should be applied in the same sense as for Fig. 1 unless the grid and cathode connections of the valve V2 of the said figure be reversed in which case the said auxiliary voltage should be applied in opposite sense relative to that for Fig. 1.

Fig. 2 of the drawings shows the characteristic of the valve V1, the line XX of the said Fig. 2 representing the point at which the said valve is operated The valve now known under the trade designation RCA 41 is a suitable valve for the valve V1 and the valve known under the trade designation RCA 58 is a suitable valve for the valve V2 and with these paiticular valves, screen grid voltages as indicated in Fig. 1, may be employed.

It is to be understood that an arrangement similar to that shown in Fig. l is employed for applying potentials to the second pair of deflecting plates.

It will be appreciated that it should be possible by suitably proportioning the parts of a circuit arrangement, in accordance with this invention, to obtain a very high degree of modulation (theoretically 100%) without distortion, i. e. the modulation. may be arranged to be substantially rectilinear throughout; the cathode ray spot should not be displaced or defocused as a result of modulation; and a standard pattern of a cathode ray tube may be employed for receiving purposes in almost any known television system of the intensity modulation type Whether the transmitter be of the cathode ray tube or of the mechanical type.

In the modification shown by Fig. 4 the valve V1 (which is shown as a pentdoe but may be a triode or a tetrode) is biased to cut off by means of voltage set up across a condenser 19' shunted by battery EG1, the input signals being applied across the terminal points I. Accordingly, there is developed across a resistance R1 in the oathode lead of the tube V1 a rectified component of the input voltage and a desired portion of this voltage is applied from tapping point 9 on the resistor R1 through resistance R2 to the control grid of a so-called variable mu valve V2. Thus the valve V2 has its gain controlled by the grid bias developed in the resistor R1. In addition, there is passed through the resistor R2 a steady direct current in order to adjust the limits of the grid voltage upon the tube V to a working range over which the grid voltage mutual conductance characteristic of the valve is rectilinear. A time base signal derived from apparatus connected at the terminal points It is also applied across the resistor R2 and the consequent time base voltage is amplifier by the valve V2. The output energy for use on one pair of electrostatic deflecting plates of the cathode ray tube (not shown) is derived by way of the connections made across the output resistance ll.

The invention is not limited to the particular arrangements described and illustrated. For example the function of the valve V1 may be performed by a diode or some other form of rectifier e. g. a metal rectifier, and types of valves other than those illustrated may be employed for V2. Provided that the rectifier at V1 is of rectilinear characteristic and the valve at V2 has a grid bias/mutual conductance characteristic which is rectilinear over the working range rectilinear correction-which is required for tubes employing electrostatic defiectionwill be obtained. Where electro-magnetie deflection of the cathode ray beam is employed rectilinear correction will not be required but instead square law correction will be required and the described arrangements should be suitably modified. For example, for square law correction the device V1 may be a square law detector and the device V2 a rectilinear amplifier (e. g. as already described) or the device V1 may be a rectilinear detector and the device V2 a valve-having a square law grid voltage/mutual conductance characteristic. The use of electro-magnetic deflection is, however, not preferred in carrying out this invention owing to the difficulties introduced by reason of the high impedance which deflecting coils present at high frequencies.

The additional. modification illustrated by Fig. is a combined compensating device and time base unit. Referring to this figure, a condenser C1 is charged from the source connected at HT+ and I-IT- through valve V2. The valve V1 is a rectilinear rectifier which feeds to the control grid of V2 a voltage derived by rectifying picture signals applied at the terminal points I, this voltage being fed to the grid by reason of the provision of a common cathode resistance R1". The charging current for condenser C1 will accordingly vary rectilinearly with the amplitude of the picture signals at the terminals I if the anode current/grid voltage characteristic of the tube V2 is rectilinear. The sweep stroke of the cathode ray tube (in the case of the line deflection component of scanning the movement of the ray in the scanning line direction) occurs during the charging of condenser C1. The device V3 is a gas filled discharge device of the nature of that known under the registered trademark Thyratron. As will be seen from an inspection of the circuit the tube V3 has its grid maintained highly negative with respect to its cathode until the voltage across condenser C2 which is charged through resistance R4 rises to the ignition potential of the neon or other glow discharge device N which is in shunt with C2 whereupon said device glow tube N flashes. When glow tube N flashes the grid of the gaseous discharge tube V3 becomes positive and the tube V3 passes current so that the condenser C1 is accordingly discharged. This discharging period gives the flyback or return stroke of the cathode ray tube and it will be appreciated that although the charging current for C1 is modulated in dependence upon the signals applied at the terminals I the frequency of the traverse of the cathode ray tube will be determined by the quantitative values in the circuit NC2R4 which constitutes an auxiliary time circuit. Adjustment of potentiometer tapping T1 adjusts the velocity of sweep and adjustment of potentiometer tapping T2 adjusts the amplitude of sweep. If desired, separate synchronizing potentials may be applied across the resistor R4.

Instead of using a Thyratron or gaseous discharge tube V3, as described above, a pair of valves arranged in a circuit as described in British patent specification No. 25,026 dated August 20, 1934, could be employed for the function of discharging the condenser and/or such an arrangement could be utilized instead of the device N in the auxiliary time base circuit.

The operation of the accompanying Fig. 5 may be summed up as follows: Picture signals are supplied at terminal points I to a rectilinear rectifier V1 from which a part of the output is fed to the control electrode [2 of the valve V2. The valve V2 has a rectilinear anode current/grid voltage characteristic and operates as a constant current device in the sense that it passes constant anode current assuming constant grid voltage. The tube V2 may, for example, be a tetrode or a pentode.

The constant current tubevzserves to charge a condenser C1'whicl1 is periodically discharged under the control of an auxiliary time base circuit including the glow tube N, a shunting capacity C2 and a resistor R4, and this circuit may be synchronized from the picture signals in the usual way. In the device represented by this Fig. 5 the control by the auxiliary time base circuit is effected by triggering a discharging device Va associated with the condenser C1. Any combination of rectifier and amplifier devices at V1 and V2 other than the rectilinear rectifier and rectilinear amplifier combination described may be employed so long as the combination is such that the anode current of tube V2 varies rectilinearly with the signal amplitude at the terminal points I.

Having now described the invention and the manner in which the same is to be operated, What is claimed and desired to secure by Letters Patent is:

1. A cathode ray tube circuit arrangement wherein a cathode ray is modulated and caused to scan a predetermined area comprising, in combination, means for modulating the anode voltage of the cathode ray tube in accordance with modulating potentials, and means for simultaneously modulating the ray deflecting forces applied to said tube to cause scanning action of the ray therein in accordance with the modulating potentials, whereby scanning speed variation resultant upon the anode voltage modulation is substantially prevented by compensating modulation of the ray deflecting forces.

2. The arrangement as claimed in claim 1, comprising, in addition, a rectifier to the input of which the modulating potentials for producing anode modulation are also applied, an amplifier, means for applying output energy from the rectifier to the amplifier to control the gain thereof, and means for controlling the deflecting forces in accordance with the amplifier output.

3. The arrangement claimed in claim 1, comprising, in addition, a multi-grid thermionic rectifier, means for applying the modulating potentials for producing anode modulation to the input electrode of said multi-grid thermionic rectifier, a multi-grid amplifying tube, means for applying the output energy from the rectifier to the input electrode of said multi-grid tube to vary the mutual conductance thereof, and means for controlling the deflecting forces in accordance with the output from said multi-grid amplifier tube.

4. The system claimed in claim 1 and comprising in addition a multi-grid thermionic rectifier having an anode and a cathode, an input and an output circuit, means for applying modulating potentials to the input circuit of said rectifier, a multi-grid thermionic rectifier having an input and output circuit, means for applying energy from the output circuit of said rectifier to the input circuit of said amplifier, an impedance connected between the cathode of the rectifier and a source of anode potential therefor, a second impedance connected to at least a portion of said first named impedance and connected to the input circuit of said amplifier, and means for supplying across said second named impedance voltages for causing deflection of the said cathode ray.

5. An anode voltage modulated cathode ray system comprising means for simultaneously modulating the anode voltage and varying the deflection sensitivity of the cathode ray in accordance with signal energy, and thermionic control means kind wherein a cathode ray is caused to scan a predetermined area which comprises means for modulating the anode voltage of the cathode ray tube in dependence upon modulating potentials, and means for simultaneously varying, also in dependence upon modulating potentials, the ray deflecting forces applied to said tube to cause scanning action of the ray therein, whereby scanning speed variation resultant upon anode voltage moduation is prevented.

'I. The arrangement as claimed in claim 6 comprising in addition, a rectifier, means for applying the modulating potentials for producing anode modulation-to the input circuit of the rectifier, an amplifier, and means for supplying the rectifier output to the amplifier to control the gain thereof whereby a deflecting force for defiecting the cathode ray in a coordinate scanning direction is derived from the amplifier output.

8. The method of controlling a cathode ray tube circuit wherein the cathode rayis modulated in intensity and caused to scan a predetermined area which comprises modulating the anode voltage of the cathode ray tube in accordance with impressed modulating potentials, and varying simultaneously in dependence upon the modulating potentials the electrical impulses causing deflection of the cathode ray for causing scanning, whereby compensation for scanning speed variation resultant from the anode modulation is provided.v

ARTHUR JAMES YOUNG. 

